"Difference Between Select Single and Select UpTo One
According to SAP Performance course the SELECT UP TO 1 ROWS is faster than
SELECT SINGLE because you are not using all the primary key fields.
select single is a construct designed to read database records with primary key.
In the absence of the primary key, it might end up doing a sequential search,
whereas the select up to 1 rows may assume that there is no primary key supplied
and will try to find most suitable index.
The best way to find out is through sql trace or runtime analysis.
Use "select up to 1 rows" only if you are sure that all the records returned
will have the same value for the field(s) you are interested in. If not, you
will be reading only the first record which matches the criteria, but may be the
second or the third record has the value you are looking for.
The System test result showed that the variant Single * takes less time than Up
to 1 rows as there is an additional level for COUNT STOP KEY for SELECT
ENDSELECT UP TO 1 ROWS.
The 'SELECT SINGLE' statement selects the first row in the database that it
finds that fulfils the 'WHERE' clause If this results in multiple records then
only the first one will be returned and therefore may not be unique.
Mainly: to read data from
The 'SELECT .... UP TO 1 ROWS' statement is subtly different. The database
selects all of the relevant records that are defined by the WHERE clause,
applies any aggregate, ordering or grouping functions to them and then returns
the first record of the result set.